Method of and apparatus for elec



June 17, 1941. E w HUBER .Re. 21,832 METHOD of AND APPARATUS FOR ELEGTRICALLY EXPLORING EARTH FORMATIONS 2 Sheets-Sheet 1 Original Filed May' 21, 1955 INVENTOR.

ATITORNEYS" 2 Sheets-Sheet 2 INVENTOR.

ATTORNEYs F. W. HUBER Original Filed May 21, 1935 June 17, 1941.

METHOD OF AND APPARATUS FOR ELECTRIGALLY EXPLORING EARTH FORMATIONS on. sun]:

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Reissued June 17, 1941 METHOD OF AND APPARATUS FOR ELEC- TRICALLY EXPLORING EARTH FORMA- TIONS Frederick W. Huber, deceased, late of Riverside;

Calif., by Schlumberger Well Surveying Corporation, Houston, Tex., assignee Original No. 2,072,950, dated March 9, 1937, Serial No. 22,533, May 21, 1935. Application for reissue February 23, 1939, Serial No. 258,110

11 Claims.

This invention has to do broadly with the exploration of earth formations by electrical methods but relates more particularly to electrical explorations conducted through drill holes-and has special application in connection with the mapping of formations penetrated by oil wells, water wells, and the like.

A broad object of the invention is to reduce the cost and time required in electrically exploring earth formations while at the same time increasing the amount of useful information obtainable from the exploration.

Various specific objects and features of the invention will be disclosed in the detailed description with reference to the drawings, which follows:

Some methods of electrical exploration of earth formations have been known for many years. Thus it is old -to determine the specific resistivity of a body of earth in situ by means of the four-terminal electrode method which was first proposed by Searles of the United States Bureau of Standards and later elaborated by Wenner and applied by Gish and Rooney and others. This method has found extensive use in locating sub-surface minerals by means of electrical measurements made on the surface of the ground. The simplest way of practicing the four-electrode method is to employ four equally I spaced electrodes disposed in a straight line and each makinggood electrical contact with the earth. The two outermost electrodes, called the input or current electrodes, are connected to an adequate current source, the circuit containing The two inner electrodes, called the potential electrodes, are not connected with the current source but have their leads connected to a potentiometer or other voltage measuring device. The current flowin between the two outermost electrodes creates anelectric field V in the earth between those electrodes and by means of the potentiometer the drop in potential in this field between the two potential electrodes is measured. Knowing (1) the value of the current flowing between the first two electrodes, (2) the distance between the electrodes, and (3) the potential difference between the potential electrodes, the specific resistivity of the earth between the potential electrodes can be deduced from the following formula credited to v P=the specific resistivity of the earth between the specific electrodes;

r=the potential electrode spacing;

E=the difference in potentials between the potential electrodes; and

l the current flowing between trodes.

It has been experimentally found by Gish and Rooney that the depth to which the sub-surface may be explored by surface measurements is approximately equal to the distance the potential electrodes are spaced apart.

In the four-electrode methods devised by Searles, the potential electrodes may'be positioned between he input electrodes or oneor both of the potential electrodes may be positioned outside of the input electrodes.

The four-electrode method of Searles and Wenner for forming specific resistivities is applicable tomaking such measurements in a vertical as well as a horizontal plane (for instance, in a mine'shaft or a bore hole) as long as the electrodes make good and sufficient electrical contact with the earth being explored. The problem of making good and sufiicient electrical contact with the formation to be explored does not exist in a bore hole where good practice dictates that the hole be kept filled with the mud-laden drilling fluid. When applying the four-electrode method to measurements in a bore hole, a ground at earth surface can constitute one of the electrodes, three electrodes being employed in the bore hole, or there can be two grounds at earth surface and two electrodes down the bore hole. In each case, however, there are four electrodes and in each case specific resistivities are computed by applying Wenners formula or a modifithe input eleccation thereof.

However, Wenners formula holds only for homogeneous earthmasses of constant specific resistivity. Where the resistivity changes from point to point, factors and coefficients have to be introduced, which make the method at best but an approximation.

electrode principle,

- galvanometer constituted by artificial resistances In all the prior methods based upon the fourthe specific resistivity is deduced from the current value flowing in the input circuit and the potential difference existing betweenthe potential electrodes. The consecutive values of the specific resistivity obtained upon moving the electrode system throughout the length of a bore hole may be plotted to provide a curve trace from which the position of the discontinuities of the strata can be located. It is important to note, however, that by this system only one geologically significant curve is obtained, e., a curve indicating the position of the discontinuities. This curve does not indicate the nature of the fluid contained in the permeable strata penetrated by the bore hole.

In accordance with the present invention it was discovered that the geological discontinuities of the strata penetrated by a bore hole can be located very accurately by a method not involving the four-electrode method and that the nature of the fluid in the porous strata can be definitely determined simultaneously by a modification of the four-electrode method without having to know the value of the current flowing in' the circuit.

In accordance with these discoveries, the discontinuities in the strata are located by recording deflections of a galvanometer in a single circuit consisting of a source of current connected between ground and an electrode movable through the bore hole. The galvanometer is located in a network in the circuit andthis network and the method in which it is used constitutes one of the novel features of the invention.

More specifically, this circuit consists of an adequate current source connected across one diagonal of a Wheatstone bridge circuit with a connected across the other diagonal. Three of the arms of the bridge are and the fourth arm is constituted by the resistance of the earth between the ground electrode and the movable electrode in the bore hole. It is old to measure resistance with a Wheatstone bridge, the well known method of procedure being to vary the artificial resistance in one or more of the arms of the bridge until no current flows through the galvanometer connected across one diagonal thereof. The unknown resistance is then computed from the values of the known resistances in the three arms of the bridge. This method gives very accurate results but has the serious objection in well surveying that it is slow, it being necessary to manually adjust the calibrated resistance elements of the bridge I until a balance is obtained, as indicated by a zero reading of the galvanometer, before the resistance can be determined. In the present system, instead of balancing the unknown resistance by a known resistance until a zero galvanometer been adjusted .to bring the galvanometer movements within a desired range). To limit the swing of the galvanometer needle to a suitable value, a large adjustable resistance is connected in series therewith. Although various methods of recording might be employed, it has been found satisfactory to use a mirror on the galvanometer suspension which reflects a light beam from a fixed source onto a moving photographic strip, the motion of which is that of the electrode traversing the bore hole, thereby tracing a curve which is some function of the changes in resistance of the formation adjacent the movable electrode in the hole at different levels in the hole. From the curve so traced, the discontinuities in the strata are located as will be more fully explained hereafter. I

The curve obtained by the procedure just described not only locates the discontinuities of the strata but in a great many fields also gives an indication of the nature of the fluid contained in the permeable strata. However, there are fields in which this indication is not positive enough to warrant definite conclusions as to the nature of the fluid contained. In accordance with the invention, it was also discovered that this limitation may be overcome by introducing a second electrode at a fixed definite distance from the first electrode in the bore hole and recording the changes in the potential drop between this second electrode and the ground surface as the electrodes are moved through the hole. erable to employ a galvanometer with a large adjustable resistance in series therewith, the galvanometer suspension having a mirror thereon which reflects light from a fixed source onto a moving photographic strip which may be the same strip upon which the first mentioned curve is pro.-

duced. The two curves when compared with each other give information which permitsdefinite conclusions being drawn as to the nature of the fluid contained in the permeable strata, even though the drill hole fluid may have pene trated some distance into the strata and displaced the natural fluids therein.

It is most important to note that although the basis of the procedure is purely empirical, the results obtained are of great practical significance.

The apparatus find procedures employed in accordance 'with the invention will now be explained with reference to the drawings, in which Figure l is a schematic view illustrating a preferred form of apparatus in accordance with the invention;

Fig. 2 is an explanatory chart showing the rec- I ords obtained from exploring a formation with the apparatus illustrated in Fig. 'l in juxtaposed relation with a map of the formation penetrated by the well;

Fig. 3 is a schematic detail view, in cross sec tion, showingthat portion of a well bore lying within an oil sand, together with the electrodes of the exploring apparatus;

Fig. 4 is a view similar to Fig. 3, but showing the exploring electrodes located within a waterbearing sand stratum;

Fig. 5 is a highly schematic diagram illustrating an alternative type of exploring cable and associated circuit to that shown in Fig. l; and

Fig. 6 is a schematic diagram showing still another alternativeelectrode and circuit arrange-- made to synchronize with To record these potential changes it is pref- The exploring apparatus propercomprises an insulated cable 3 containing a pair of electrical conductors, each of which is connected at the lower end of the cable to separate electrodes 4 and 5, respectively. The cable 3 will have a total length somewhat in excess of the depth of the hole to be explored, which cable is normally wound upon a drum '6 positioned adjacent the top of the well. The cable is unwound from the drum 6 to lower the electrodes 4 and 5 into the hole, and is rewound upon the drum to raise the electrodes. Between the drum 6 and the hole the cable passes around a spool or drum 1 which functions to move a recording tape in synchronism with the movements of the electrodes 4 and a shaft l4 which turns a spool l5 to wind a photographic film ||i, the film being supplied from a feed spool H.

The two conductors Within the cable 3 are connected to the electrodes 4 and 5, respectively, at

the lower end of the cable and are connected at the opposite end of the cable to a pair of contact segments l8 and I9, respectively, on the exterior of drum 6, which segments are contacted by brushes 20 and 2|, respectively. Segment l8 connects to the conductor associated with electrode 4, and segment [9 is connected to the conductor associated with electrode 5.

In operation a potential is impressed between ground and the electrode 4 and provision is made for indicating variations in the resistance between these points. To this end the brush 2|] is connected to one terminal D of a Wheatstone bridge 22, an adjacent terminal A of which is connected to ground by a conductor 23. The conductor 23, the earth path from conductor 23 to the electrode 4, the cable conductor connected to electrode 4, the segment I8, the brush 20 and the conductor 24 extending between brush 20 and the terminal D constituting the unknown arm of the bridge 22, equal resistances a and b constitute the fixed arms of the bridge between terminals A and B and B and C, respectively, and an adjustable resistance constitutes the arm of the bridge between terminals C and D. A constant potential is impressed across the terminals A and C by a source of potential 25 which is shown as a battery but may be a generator. The poles B and D of bridge 22 are connected to a recording voltmeter which may consist of a mirror galvanometer 26 connected in series with an adjustable resistance 21.

Although referred to as a mirror galvanometer, the device 26 is preferably a relatively rugged milliammeter, having a mirror mounted on its moving coil. This mirror is adapted to reflect a beam of light from a lamp 28 onto the sensitive film Hi to produce (after the film has been developed) a record 29.

The segment l9 and associated brush 2| are connected through the second conductor of the cable 3 to the electrode which, as previously stated, is electrically distinct from and spaced a predetermined distance from the electrode 4. This electrode is connected through segment I9, brush 2|, a conductor 30, adjustable resistance 3|, and a second mirror galvanometer 32 to ground over a ground conductor 33. Galvanometer 32 functions to'indicate variations in the potential drop between ground and electrode 5,

which resultsfrom the earth current produced by the battery 25 between earth and the electrode 4. Galvanometer 32 is so positioned as to reflect a beam of light from the lamp 23 onto the film l6 at a point substantially juxtaposed to the beam from the galvanometer 26.

The electrodes 4 and 5 are first positioned within the hole adjacent the upper or surface end thereof, which portion of the hole is usually cased, as indicated at 35, and the bridge 22 is adjusted by adjustments of the resistance C until the galvanometer 26 produces a zero registration at a desired point on the film l6. This adjustment of resistance C to bring the galvanometer to the arbitrary zero reading is made without any. reference to the value or direction of the current flowing through the galvanometer-.

there being no attempt whatever to balance the Wheatstone bridge and thus bring the galvanometer to zero indicating equal potential at B and D. Likewise the resistance 3| is varied to produce a predetermined desired deflection of the galvanometer 32 so as to position the beam reflected therefrom at a desired point on the film. Thereafter the cable is lowered into the hole at a uniformrate of speed to traverse the hole with the electrodes 4 and 5. At the same time the movement of the cable 3 rotates the drum 1 to cause the film Hi to be moved past the beams of light reflected thereon from the galvanometers 2B and 32, respectively, at a fixed speed relative to the movement of the cable.

Since the variations in total resistance of the circuit through the earth formation between the ground conductor 23 and the cable electrode 4 is largely dependent upon the resistance of the particular formation immediately adjacent the electrode 4, the resistance of that arm of the bridge between A and D varies with the movement of the electrode 4 past difierent formations. variations in the ratios of the potentials existing across the different arms of the bridge and cause variations in potential between the poles B and D, which variations in potential produce deflections in the galvanometer 2G causing the beam of light reflected therefrom onto the film 6 to describe an irregular line upon the film.

The current flowing through the earth between the ground 23 and electrode 4 sets up an electric field in the earth, the relative intensity of the field in various points in the earth depending upon their distances from the electrodes and the various electrical properties of the strata adjacent those points. As the electrode 4 traverses the well bore, variations in the electrical properties of the strata traversed will therefore cause variations in the electrical field surrounding the electrode. In accordance with the discoveries of Gish and Rooney, these variations may be indicated by the use of the four-electrode system for a depth equal to the spacing of the potential electrodes.

Therefore the variations in the electrical properties of the ormation between the hole and a point at a distance from the hole I substantially equal to the distance between the electrodes 4 and 5, causes a change in the potential between earth and the electrode 5, which changes in potential areimpressed upon the galvanometer 32, causing the latter to describe an irregular line 40 upon the film I6. To provide for adjustment of the amplitude of movement of the galvanorm eters 26 and 32, respectively, the sensitivities of These variations in resistance cause .by the galvanometer 28, and, at the same time,

a record of changes in the potential of the electrode 5 (which is a measure of the resistivity of the formation at a distance from the hole substantially equal to the distance of the electrodes 4 and 5), is made by the galvanometer 32. The

fact that the recordings are made entirely au- .tomatically is of the utmost importance for the reason that it permits the electrodes to be run through the hole rapidly, thereby reducing the total time required for exploration and it also precludes any possibility of error due to the human element, such as frequently result where readings are taken by an observer and manually recorded. I

The adjustable resistance 21 in series with the galvanometer 26 serves to regulate the total throw of the galvanometer needle from its established zero position; This makes it possible to vary the sensitivity of the galvanometer over a wide range and thus obtain any desired amount of detail on the curve trace produced. This is of great practical importance as the diameter of the hole and salinity of the drillng fluid in the hole influence, the sensitivity to a marked degree. For example, if the sensitivity is ad justed to give good detail in a bore hole 6 inches in diameter, the same sensitivity adjustment when measurements are being made in a 12 inch hole would give little or no detail and the total throw of the galvanometer would be so small that the breaks in the curve would be more or less masked and the location of the discontinuities in the strata would not be indicated deflnitely. In other instances the salinity of the fluid in the bore hole might be such as to give too great a throw to the galvanometer needle, causing most of the curve to fall outside the limits of the recording strip or fllm. In practice, the sensitivity is adjusted by adjusting the. resistor 21 immediately after the electrodes are lowered below the cased portion of the hole. at

' which level the bridge is adjusted, as previously described.

To prevent polarization effects, it is preferred to run the electrodes through the hole at a speed of not less than 30 feet per minute. However, it is usually desirable, in order to conserve time, to run the eletrodes at substantially higher speeds, and speeds up to 600 feet per, minute have been employed with very satisfactory results.

' It is to be understood that measurements may also be taken by moving the electrodes from the bottom of the hole up to the surface while making recordings instead of, or in addition to, making recordings while lowering the electrodes.

The records produced in the manner described are preferably .made on film which is calibrated longitudinally in accordance with the distance traversed by the electrodes in the well hole. A portion of such a record is indicated in Fig. 2, and there is shown juxtaposed to the record a map of actual earth formations traversed by the electrodes. The curve 29 represents variations in resistance between earth and the current electrode 4 and the curve 40 represents variations in potential between earth and the electrode 5.

The galvanometer 26 is preferably so poled that deflections to the right represent increased resistance adjacent the electrode 4 and the galvanometer I2 is so poled that deflection of curve 40 to the right represents a decreased potential impressed upon galvanometer 32, which is indicative of an increased resistance in the formation at a distance from electrode-i equal to the distance between electrodes 4 and 5.

From an inspection ofFig. 2, it will be observed that at a depth between 3800 feet and approximately '3850 feet, where the electrodes were passing through a sand and shale formation, the curve 29 indicated a relatively low resistance and the curve 4| likewise indicated a relatively low resistance at a short distance from the hole. Thereafter, between the depths of substantially 3850 feet and 3900 feet the electrodes passed an oil sand which was indicated by rather violent deflections of the curves 3! and 40 to the right,

- indicating a higher earth resistance. Thereafter,

from a depth of 3900 feet tosubstantially a depth of 4000 feet, the electrodes traversed a shale formation showing a relatively low resistance both adjacent the bore hole and at a short distance therefrom. Thereafter, from a depth of approximately 4000 feet to a depth of approxi-.

mately 4100 feet, the electrode 'traversed an oil sand which increased the resistance adjacent both electrodes as shown by deflection of curves 3! and 40 to the right. From approximately 4100 feet to 4160 feet the electrodes again traversed a shale formation which is indicated on the curves 3! and 40 by relatively low resistance.

Below the last mentioned shale formation and extending substantially from .a depth of 4165 to 4230 feet is an oil sand formation which would not be detectable from curve 29 alone. Thus it will be observed that in the oil sand formation which occurred at a depth between 3800 and 3900 will be observed that whereas the curve 29 indie cated a reduced resistance between the last mentioned depth, the curve 40 indicated an increased resistance. In other words, the two curves read together indicate that the-resistance closely ad- .Iacent the bore hole decreased, whereas. resistance at a distance from the bore hole increased. It has beenfound by experiments that the explanation of such an apparently anomalous result is found to be due to the fact that the. pressure of the drilling fluid in the drill hole sometimes forces the oil in anoil bearing sand back away from the hole for a substantial distance the drilling fluid having relatively high conductivity replacing the oil in the sand immediately adjacent the hole.

This condition is illustrated in Fig. 3, in which the. hole I is shown passing through a shale formation 50 into an oil sand 5|. The drilling'fluid 2 within the bore holel has permeated the oil sand to the distance H, forcing the oil back away from the hole. Whenever the curves 2! and Ill separate, as shown in Fig. 2 adjacent the 4200 foot depth, it is strongly indicative of an oil sand which has been permeated to a greater or less extent by the drilling fluid in the well, the different recordings resulting from the fact that the resistance in the formation at a distance from the hole is substantially greater than the resistance of the formation closely adjacent the hole. This divergence enables the operator to distinguish a permeated oil sand from a water sand. Thus there is shown in Fig. 4 a water sand formation 52 between shale formations 53 and 54 which is likewise permeated to av greater orless extent by the drilling fluid 2 in the bore hole I. In this instance the resistance at a distance from the bore hole may be slightly greater than the resistance immediately adjacent the bore hole, but the resistivity at both points will be substantially less, for instance, than in the shale formations 53 and 54 immediately above and immediately below the water sand. In short, a divergence of the curves 29 and an indicates a divergence in the resistance characteristics of the formation adjacent the hole from the resistance characteristics of the formation at a distance from the hole equal to the spacing between the electrodes.

A formation, therefore, suchas that shown at 52 in Fig. 4, would cause a deflection of both curves 29 and 4D to the left and the operator would be definitely apprised of the fact that the formation was not an oil or gas sand (oil and gas sand both having the characteristic of having a relatively high electrical resistivity).

The distance between the electrodes 4 and 5 usually varies between 2 to feet, the optimum spacing depending uponthe nature of the formation and the lateral distance to which the well fiuid usually permeates an oil or gas sand. The current electrode 4 in Fig. 1 should be of substantial area, say from to 100 square inches, in order to reduce the local resistance to a sufficient extent. The secondary electrode 5, however, may be very small as it does not have to carry appreciable current. The constants of the bridge 22 should be roughly proportioned to the resistances to be measured, and it has been found satisfac tory to make resistances a and b 50 ohms: and

the variable resistance 0 from zero to ZOO ohms.

The voltage of the battery. or other source 25,

is usually between 100 and 200 volts which, with the resistance value specified in the bridge arms applies a voltage across the terminals A and B of from to 85 volts. The resistances 2 1 and 3| are preferably variable from approximately 100 to 200,000 ohms.

Various modifications may be made in the circuit and apparatus of Fig. 1. Thus instead of coordinating the movement of the film IS with that of the cable 3 by the mechanical linkage shown in Fig. 1, the cable and film may be caused to move in synchronism with each other by any known synchronizing mechanism, such as the well known Selsyn motors.

Also various modifications may be made in the electrical circuit shown. Two such modifications any suitable point. In Fig. 5, the earth connection is made by employing a cable 3' having an external metallic armor ill to protect the cable from mechanical abrasion and employing this armor as the ground connection by connecting the conductor 23 directly thereto. The conductor 33 from galvanometer 32 may also be grounded on the armor 60, if desired, although it is generally preferable to connect this conductor 33' to an independent ground.

It should be noted in this connection that it is not as important to have a low resistance ground connection for conductor 33' as for conductor 23' since the latter carries a substantial current whereas the current in the circuit including conductor 33 is of negligible value. The metallic armor 60 of course must be terminated a substantial distance above the electrodes 4' and 5' in order to make the resistance of the circuit including the electrode 4 sensitive to changes in the resistivities of the adjacent formation.

In Fig. 6 an arrangement is disclosed employing three electrodes connected to the surface by a cable having three electrically distinct conductors GI, 62, and 63, respectively. Conductors 62 and 63 are connected directly to electrodes 4" and 5" which correspond to electrodes 4 and 5 in Fig. l, these electrodes being connected to the bridge similar to the connections in the circuit of Figs. 1 and 5. Thecable conductor 6|, however, connecting to the additional electrode 64, connects to the conductor 23' and takes the place of the earth connection .in Fig. 1 and the cable armor in Fig. 5. The potential impressed by thebattery 25" across the electrodes 64 and 4 setting up a field therebetween, the strength of which is a function of the resistivity of the formation adjacent those electrodes. In Fig. 6, as in Fig. 5, the galvanometer 32" is connected directly to an independent ground from conductor 33".

Various other differences in the detail construction and operation of the circuits described may be made by those skilled in the art without are shown in Figs. 5 and 6, respectively, in which elements corresponding to those in Fig. I bear the same reference numerals with the single prime mark added in the case of Fig. 5, and the double prime mark added in the case of Fig. 6. The showing of the film and the mechanism syn.- chronizing the film with the cable, together with the cable, are omitted in Figs. 5 and '6 to simplify the drawings.

The arrangement shown in Fig. 5 differs primarily from that shown in "Fig. 1 in the method of making the ground connection to conductor 23': In Fig. 1 this ground connection was made by means of an electrode buried in the ground at departing from the spirit of the invention, and the invention is to be limited, therefore, only as set forth in the appended claims.

What is claimed, is:

1. In earth exploration, the method of determining the nature of the natural fluid contained in a porous formation traversed by a bore hole containing a conductive fluid, which comprises measuring and comparing variations in the electrical resistivity of the formation parallel and close to the bore hole where the formation is permeated with the conductive fluid of the bore hole and parallel to but spaced from the bore hole a distance greater than the distance of penetration of the bore hole fluid from the bore hole into the formation.

2. The method of determining the relative character of fluids in porous strata traversed by a bore hole at points closely adjacent the bore hole and at a predetermined distance away therefrom, respectively, the bore hole containing a relatively conductive driling fluid, which comprises moving a pair of electrodes which are spaced apart vertically approximately said predetermined distance along said hole through the drilling fluid therein, impressing an electric potential between one of said electrodes and ground at a point substantialy distant from said two electrodes, whereby the current through said one electrode is approximately proportional to the formation resistance immediately adjacent said one electrode, measuring the direction of deviation from a selected arbitrary value of said current through said electrode at all levels in said bore hole, and measuring the direction of variation from a selected arbitrary value of the potential difference created by said current between the other of said electrodes and ground at a point substantially distant from said electrodes, at all levels, the relative direction of deviation from said arbitrary values of said curbore hole whereby an electric field is produced in the formation surounding the said electrode, measuring the direction and approximate relative magnitude of variations of said current from an arbitrary reference value, said variations resulting from changes in the formation closely adjacent the moving electrode, simultaneously measuring the direction and approximate relative magnitudes of variations from an arbitrary reference value of the potential of said electric field at a moving point in said =bore hole spaced a fixed distance from said moving electrode, for comparing and contrasting the variations in said current and said'potential respectively, at differ than that aforementioned, and comparing the said measured electrical characteristic variations.

5. In earth exploration the method of determining for diiferent depths of investigation the nature of the strata traversed by a bore holeand comparing the eifects of the flow of said current in the' part of the formations through one depth of investigation relatively close to the drill hole, and in the part of the" formations through a' greater depth of investigation than that aforementioned.

8. The method of exploring for diiferent depths of investigation earth formations traversed by a drill hole which comprises the steps of moving an electrode through the drill hole, passing current through a circuit including the electrode, the earth and a grounded point, and

which comprises passing a current between; ground and a moving electrode traversing the which comprises the steps of measuring simultaneously variations in the resistivity of the formation through one depth of investigation relatively close to the bore hole, and variations in the resistivity of the strata through a greater depth of investigation than that aforementioned,

and comparing the said measured resistivityvariations.

6. In earth exploration, the method of determining the nature of the natural fluid contained in a porous formation traversed by a bore hole containing aconductive fluid, which comprises measuring and comparing variations in an electrioal characteristic of the formation parallel and close to the bore hole where the formation is permeated with the conductive fluid of the bore hole and parallel to but spaced from the bore hole a distance greater than thedistance of penetration of'the bore hole fluid from the bore hole into the formation.

'7. The method of exploring for different depths of investigation earth formations traversed by a drill hole which comprises the steps ofpassing current into the formations surrounding the drill hole, and simultaneouslymeasuring 7 simultaneously measuring and comparing the efl'ects'of the flow of said current'inth'e'earth' in 'a zone through one depth of investigation relatively close to the electrodea-nd in-a' zone through agreater depth of investigation than that aforementioned.

9. The method of' exploring for different .depths of investigation earth formations traversed by a drill hole which comprises the steps of moving an electrode through the drill hole, passing current through a circuit including the electrode, the earth and a grounded point, and simultaneously measuring and comparing variations in resistivity in the earth in a zone through one depth of investigation relatively close to the electrode and in a zone through a greater depth of investigation than that aforementioned.

10. Apparatus for 1 exploring the formations traversed by a drill hole comprising an electrode movable through the drill hole, means for causing a current to flow through said electrode into the formations, means for measuring the resistivity of the formations in a zone close to the electrode, a second electrode in the drill hole, spaced apart from said first electrode and movable therewith, and means for measuring the Potential difference between said second electrode and a grounded point to indicate the resistivity of the formations in a zone spaced from said first electrode.

11. The method of determining simultaneously in a bore hole containing conducting fluid, the relative nature of the strata traversed at points closely adjacent the bore hole and at a predetermined distance away therefrom, which comprises the steps of moving along the holea pair of electrodes spaced apart vertically approxi- -mately said predetermined distance, impressing an electric potential between one of'said elec trodes and a ground point substantially distant from said two electrodes, whereby the current through said one electrode is a function of the formation resistance immediately adjacent said one electrode, measuring the direction of deviation from a selected arbitrary value of said'cur- 'rent through said electrode at all levels in said current between the other of saidelectrodes and .from said bore hole, respectively.

a ground point substantially distant fromsaid electrodes, at all levels, the relative direction of deviation from said arbitrary values of said cur rent and potential, respectively, at the levels of said strata affording indications of Lthe relative nature thereof, at points adjacent-and spaced .SCHLUMBERGER WELL SURVEYING CORBORATIQN; Assignee, by mesne assimzmentaof Frederick W Huber, Deceased,

By EUGENE G. LEONARI JON, Vice-Pfesideni. 

